This invention relates to production of corrosion-resistant spinnerets and, in particular, to spinnerets suitable for spinning filaments of poly(p-phenylene terephthalamide) from a viscous solution of the polymer in 98 to 100 percent sulfuric acid.
Blades U.S. Pat. No. 3,767,756 discloses production of filaments having remarkably high tenacities from poly(p-phenylene terephthalamide) and related wholly aromatic polyamides (aramids) by spinning a viscous solution of the polymer in 98 to 100 percent sulfuric acid through spinneret capillaries 2 to 4 mils (0.05 to 0.10 mm) in diameter. Chlorosulfuric and fluorosulfuric acid are also disclosed as suitable solvents. When such highly corrosive solutions are extruded through filament-forming capillaries in spinnerets composed of customary materials, corrosion soon makes the spinnerets useless. An initial effect is that the originally sharp edges, defining the outlet peripheries of the capillaries, become dulled and rounded. This causes erratic non-uniformities of denier in the spun filaments, and threadline breaks occur at increasing frequency.
Tantalum has excellent corrosion resistance and has been proposed for spinnerets used in spinning rayon by the viscose process. Austin U.S. Pat. No. 1,791,785 teaches that tantalum is too hard for properly drilling spinneret holes unless it has been softened or annealed, but then the holes are likely to be deformed by subsequent use of the spinneret. Austin proposes coating the spinneret with an electrolytic film. British Pat. No. 702,936 mentions previous attempts to surface-harden completed tantalum spinnerets by heating them in air, oxygen, nitrogen or carbon monoxide, but states that such treatments impair the quality of the spinning passages to such an extent as to cause them to get rapidly blocked during spinning. Another difficulty with annealed tantalum is that it is a yield strength of less than 30,000 pounds per square inch and an elongation of at least 20 percent, i.e., is quite ductile. Particularly for the present purpose, an excessively thick tantalum spinneret would be required to avoid bulging at the high spinning pressures used. Optimum fiber properties are also difficult to obtain because of the length of spinning passages through a thick spinneret.
Hull U.S. Pat. No. 2,965,924 proposes forming spinneret holes in thin sheet metal, which may be a noble metal, drilling holes in a corresponding pattern in a spinneret blank, plating the blank with copper or silver, assembling the punched sheet metal with the plated blank, and brazing the assembly together. There are several difficulties with this procedure. (1) Precise alignment of holes produced in the separate parts is too difficult. (2) Excess of the copper or silver brazing metal can partially or completely block spinneret capillaries. (3) Deformation of the assembly occurs during brazing. (4) Discontinuities in the brazing metal allow leakage to occur between the sheet metal and the blank. Ogden et al. U.S. Pat. No. 3,279,284 proposes avoiding the use of brazing metal by instantaneous welding, e.g., explosively bonding the parts together by detonating a sheet of explosive material. However, adequate bonding in this manner inevitably causes some deformation of the relatively soft face layer containing the spinneret capillaries; repolishing the face to a flat surface then introduces non-uniformities in the lengths of the spinneret capillaries which cause objectionable denier variations in the filaments produced.